1. Field of the Invention
The invention relates to control of a movable member and control of object conveyance, and in particular to determining a displacement of the movable member and object.
2. Background
In order to increase productivity, image forming devices such as printers and copiers are required to further speed up while keeping high print quality. To respond to such a request, the devices need further highly accurate control of movable members and sheet conveyance; The movable members include conveyance rollers and a rotatable image carrier such as a photoreceptor drum and an intermediate transfer belt. One idea for an increase in accuracy of the control is to more precisely and rapidly determine a shift or velocity (hereinafter, referred to collectively as “displacement”) of a target, e.g. one of the movable members and sheets.
In highly precise and rapid determination of a displacement, optical detectors have an advantage, and in particular, those of a type with an image sensor are promising. See, for example, JP 2015-068809, JP 2013-144604, JP 2011-093241, and JP 2002-071310. A detector of this type uses an image sensor, such as complementary metal-oxide semiconductor field effect transistors (CMOS) or a charge coupled device (CCD), to capture a continuous series of images of a target, and from the differences between two of the images, calculate a displacement of the target. In general, a detector of the type that captures from the images finer shapes of the target achieves a higher degree of precision.
Recently, use of speckle pattern has been attracting attention as a promising means for determining a displacement with a higher degree of precision. “Speckle” means interference of coherent light diffusely reflected by irregularities in the size not lower than the wavelength of the light. See, for example, “A Speckle Reduction Filter Using Contrast Information,” Tomoyuki MITAMURA, Hideo KITAJIMA, Tomoaki SHIRAKAWA, and Yoshihiko OGAWA, Bulletin of the Faculty of Engineering, Hokkaido University, No. 164, (Hokkaido University, May 28, 1993) pp. 49-54, http://hdl.handle.net/2115/42361; “Laser Speckle Kenkyu no Tenkai (in Japanese),” Toshimitsu ASAKURA and Jun UOZUMI, Research for Electronic Science, Vol. 4 (Research Institute for Electronic Science Hokkaido University, February 1997), pp. 3-12, http://hdl.handle.net/2115/24368. This interference of light provides the distribution of amounts of the reflected light with “speckle pattern,” i.e. distortions in striped or dotted pattern that corresponds to the shape of fine irregularities on the reflecting surface. The speckle pattern is used in displacement determination as follows. See, for example, JP 2015-068809 and JP 2013-144604. A light emitted from a semiconductor laser is used to capture images of a target. In general, this light has a wavelength of 0.1 μm-1 μm, and the target has a surface roughness of 1 μm or more, and thus, speckle pattern appears in each image of the target. Since the speckle pattern reflects the fine structure of the target's surface, the speckle pattern of each image has a correlation with that of the next image, and a coefficient of the correlation is distributed in each image with a peak. Caused by the moving target, the peak is shifted in the next image from the location where the peak would appear if the target were at rest. The ratio of the shift amount of the peak to the magnification of the optical system on the side of the image sensor is used to determine the displacement of the target. Such a use of speckle pattern can increase precision of the displacement to the surface roughness of the target, i.e. to micrometer precision.